Optical isolator with encapsulation

ABSTRACT

An optical isolator is fabricated upon a lead frame having an LED section which is rotated 180° to position the LED over the photodiode. Prior to rotation both the LED and the photodiode attachment portions of the leads are down set a predetermined amount to fix the size of the isolation gap between the LED and the photodiode. Dielectric sheets are attached to the outer surfaces of the LED and photodiode leads and an optically transmissive resin is injected therebetween to form a light guide and to encapsulate the LED and photodiode dice and their associated bond wires. In an alternate embodiment a sheet is attached to the outer surface of one lead and a dielectric sheet thereagainst is positioned at a tilted angle between the LED and the photodiode.

This is a division of application Ser. No. 07/365,148 filed on Jun. 9,1989, now abandoned, which was a division of application Ser. No.07/076,094, filed on Jul. 21, 1987 and now issued as U.S. Pat. No.4,863,806, which was a division of application Ser. No. 06/748,520,filed on Jun. 25, 1985 and now issued as U.S. Pat. No. 4,694,183.

BACKGROUND AND SUMMARY OF THE INVENTION

An optical isolator may be used in an electrical circuit to allow signalpropagation in a forward direction while maintaining voltage and currentisolation between the input and the output of the optical isolator. Suchan isolator includes a light emitting diode (LED) at the input, aphotodiode at the output and an optically transmissive high breakdownvoltage isolation gap separating the LED and the photodiode. In order toachieve optimum isolator performance the size of the isolation gap andthe alignment of the LED and the photodiode must be maintainedaccurately during manufacture.

One prior art attempt to automate optical isolator manufacture,described in U.S. Pat. No. 4,446,375 and shown in FIGS. 1A-C hereof, hasbeen to align the LED and the photodiode during fabrication by foldingthe LED lead and the separate LED bond lead into place over the detectorin two ninety degree steps. Although the use of such a folded lead frametechnique decreases the assembly time for each isolator a complexfolding apparatus is required to make the two 90° folds which lie alongdifferent axes and errors in the placement of the folds may affect theaccuracy of the LED-photodiode alignment. During folding, damage to thebond wires may occur because the LED lead and the LED bond lead are notsecurely attached together and because the dielectric sheet between theLED and the photodiode dice is free to rest against and damage the LEDand photodiode bond wires. As additionally described in U.S. Pat. No.3,925,801 and shown in FIG. 2 hereof, the size of the isolation gap inmany prior art optical isolators is determined by the thickness of adielectric sheet located between the LED and the photodiode. Theinsertion of such a sheet during manufacture has often resulted in bondwire damage.

In accordance with the illustrated preferred embodiment of the presentinvention, an optical isolator is manufactured simply and repeatablyusing standard form lead frames. Prior to die attachment and wirebonding of the optical isolator, the LED and photodiode attachmentportions of the lead frame are displaced to a common plane located belowthe plane of the remainder of the lead frame. The entire LED section ofthe lead frame is then rotated 180 degrees about a pair of hinge pinsuntil it is coplanar with the photodiode section of the lead frame.After rotation is completed the hinge pins are stamped to eliminate thetendency of the LED section of the lead frame to return to the previousnon-rotated position. A clear resin is injected between two dielectricsheets attached to the outer surfaces of the LED and photodiode leads toform a light guide between the LED and the photodiode. The opticalisolator is encapsulated and the protruding leads are cut and bent tothe desired final package shape. In an alternate preferred embodiment ofthe present invention, a first dielectric sheet is attached to the outersurface of either the LED or the photodiode lead and a second dielectricsheet is positioned between the LED and the photodiode dice. The secondsheet rests upon (and adheres to adhesive on) the first sheet andextends between the dice at an angle so that damage to the dice and tothe bond wires is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C show a prior art optocoupler having a folded lead frameconstruction.

FIG. 2 shows a prior art optocoupler having a dielectric sheetcompressed between the LED and the photodiode.

FIG. 3 shows an optical isolator lead frame in accordance with thepreferred embodiment of the present invention.

FIGS. 4A-B are a flow chart of the optical isolator assembly steps inaccordance with the preferred embodiment of the present invention.

FIGS. 5-11 show various stages in the assembly of the optical isolatorin accordance with the flow chart of FIG. 4.

FIG. 12 shows an alternate preferred embodiment of the present inventionin which a dielectric sheet is positioned between the LED and photodiodedice at a tilted angle.

FIG. 13 is a flow chart of the assembly steps of the alternate preferredembodiment shown in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference numbers are assigned to elements in the figures to aid inunderstanding the invention. Like reference numbers in multiple figuresrefer to the same element and this element may not be discussed in thediscussion of each figure in which the element appears.

FIGS. 1 and 2 show optocouplers which are fabricated according to theprior art. FIGS. 1A-C illustrate two 90° folding steps, described inU.S. Pat. No. 4,446,375, which have been used in the manufacture ofprior art optocouplers to align the LED and the photodiode. FIG. 2 showsan optocoupler, described in U.S. Pat. No. 3,925,801, which uses thethickness of a single dielectric sheet compressed between the LED andthe photodiode dice to establish the size of the isolation gap.

FIG. 3 shows a lead frame 1 which is used in the assembly of an opticalisolator in accordance with the preferred embodiment of the presentinvention. The lead frame 1 is 0.010 inch thick and is fabricated fromalloy 42 (42% nickel, 58% iron) in a well known manner. The hatchedregions are plated with gold to facilitate later die attach and wirebond operations and for the sake of clarity these plated regions are notshown in the other Figures. Two side rails 3 and 5 provide longitudinalstability for the lead frame 1 which is approximately 8.4 inches inlength and one inch in width. Repeated tie rails 21 and 23 providelateral stability to the lead frame 1.

Ten each alternating LED sections 11 and photodiode sections 13 arelocated on lead frame 1 and each individual section is divided aboutarms 17 and seats 19 into identical left and right halves. The LEDsections 11 are attached to side rails 3 and 5 by 0.014 inch wide hingepins 31, 33 and the photodiode sections 13 are attached to side rails 3and 5 by dam bar ends 35, 37. During fabrication each left pair of LEDsections 11 and photodiode sections 13 will be combined to form oneoptical isolator Each right pair will be combined into one opticalisolator in a like manner. A total of twenty optical isolators instandard integrated circuit 8-lead packages will be fabricated upon thesingle lead frame 1.

FIGS. 4A-B are a flow chart of the various assembly steps which areperformed during the manufacture of an optical isolator in accordancewith the present invention. The individual assembly steps for a singleoptical isolator 103 are discussed below in connection with FIGS. 5-11.During automated assembly all of the optical isolators on the lead frame1 would be fabricated sequentially.

FIG. 5 shows the lead frame 1 after performance of step 41 in whichattachment portions 81 and 83 are down set below the plane of side rails3 and 5. The down set operation may easily be performed using well knownstamping techniques and the amount of down set is predetermined to setthe size of the isolation gap. For an isolation gap of 0.010 inch in thepreferred embodiment, each of the attachment portions 81, 83 was downset a distance of 0.019 inch although this downset distance may varywith the thickness of different LED and photodiode dice. The seat 19 isdown set by an amount equal to the thickness of the arm 17, i.e., 0.010inch, to set coplanar alignment of the LED section 11 with thephotodiode section 13 after rotation by allowing the arm 17 to restagainst the down set area of the seat 19. After rotation is completed,the two sections 81 and 83 will be located equidistantly above and belowthe plane of the side rails 3, 5.

FIG. 6 shows steps 43 and 45 in which the die attach and wire bondoperations are performed upon lead frame 1 in a conventional manner. AnLED die 91 and an optional buffer die 93 are attached to an LED lead 95and a photodiode die 97 is attached to a photodiode lead 99. Variousbond wires are attached to the other leads shown as required. The LED 91is a high speed gallium arsenide phosphide device of the type describedin U.S. Pat. No. 3,836,793 which is incorporated herein by reference.The LED 91 has a thickness of approximately 0.006 inch and thephotodiode 97 has a thickness of approximately 0.012 inch. Side rails 3and 5 are also shown.

FIG. 7 shows the first of the combination steps 47 which are outlined ingreater detail in the flow chart of FIG. 4B. For the sake of clarity,only the left section of the lead frame 1 is shown in FIGS. 7-9. Incombination operation step 61, the tie rail 21 of LED section 11 issevered from both of the side rails 3 and 5 (not shown). This leaves theLED section 11 attached only at the laterally aligned hinge pins 31, 33(not shown) and longitudinal rotation about the hinge pins is possible.The dam bar ends 35 and 37 (not shown) of the photodiode section 13 arenot severed and, consequently, the photodiode section 13 remains rigidlyattached to the side rails 3 and 5.

In step 63, the LED section 11 is rotated approximately 15° upwardsabout the hinge pins 31 and 33. The rotational movements of steps 63-69may be made in a single step or in multiple steps as required for theparticular combination apparatus used. Since the lead frame 1 isconfigured for a standard 8-lead package, standard integrated circuitprocessing equipment may be used. Of course, persons of ordinary skillin the art will be well able to reconfigure the lead frame 1 to allowrotation of the photodiode section 13 instead of rotation of the LEDsection 11.

FIG. 8 shows step 65 in which the LED section 11 is rotated toapproximately 90° relative to the plane of the side rails 3, 5 (notshown). In step 67 (not shown), the LED section 11 is rotated toapproximately 165°. Of course, the entire 180° rotation could beperformed in a single step instead of in the multiple steps used in thepreferred embodiment. Hinge pin 31 is also shown.

FIG. 9 shows step 69 in which pressure pads of the combination apparatuspress the LED section 11 to a fully 180° rotated position coplanar withside rails 3, 5 and photodiode section 13. At this point, the arm 17 isseated against the down set area of the seat 19 and damage to the twodice may be avoided if the lead frame 1 is accidentally bumped. The arm17 and the seat 19 may be spot welded together to provide additionalstability during transfer molding. Since, unlike certain prior artdevices, the size of the isolation gap is not set by compression of adielectric sheet between the dice 91 (hidden), 97 (hidden) damage to thedice 91, 97 does not occur.

In order to prevent rotational spring back of the LED section 11 fromthe 180° rotated position the two hinge pins 31, 33 (not shown) arestamped and thereby flattened. The width of each of hinge pins 31, 33(not shown) must be greater than the thickness of side rails 3 and 5 sothat swaging can occur. But, the hinge pins 31, 33 (not shown) widthsmust not be so great that excessive material cold flows during stampingthereby causing buckling or deformation of portions of the lead frame 1.In the preferred embodiment, widths in the range of 0.012 to 0.016 inchwere used. Hinge pin 31 is also shown.

FIG. 10 shows a close-up cross sectional view of the optical isolator103 after completion of the step 47. The LED 91 is positioned correctlyover the photodiode 97 across an isolation gap 101. The size of theisolation gap 101 is determined in step 41 by the amount of down set ofattachment portions 81, 83 (both shown in FIG. 5) and seat 19 (shown inFIG. 5) and by the thicknesses of the dice 91 and 97. The leads 95 and99 are coplanar which permits the use of standard transfer moldingapparatus in step 53 and eliminates flashing on the bond leads duringmolding. Buffer die 93 is also shown.

FIG. 11 shows steps 49-55 in which final packaging of the opticalisolator 103 is performed. In step 49, two dielectric sheets 111, 113are attached to the outer surfaces of leads 95 and 99. Sheets 111, 113may be made from any nonconductive material such as 0.0028 inch thickcommercially available DuPont Company Kapton tape which is cut to 0.100by 0.100 inch in size and which has an adhesive coating on one side. Instep 51, an optically transmissive silicone resin such as Dow CorningCompany's Hipec Q1-4939 semiconductor protective coating is injectedinto the isolation gap 101 between sheets 111 and 113 to form a lightguide 115. By using surface tension forces, the sheets 111, 113 limitthe resin flow to the desired location and preclude structural weakeningof the optical isolator 103 which could be caused by excessive resinflow. The resin effectively encapsulates the dice 91, 93, 97 within thelight guide 115 and thereby reduces the risk of damage to the opticalisolator 103 during thermal shock or temperature cycling.

In step 53, standard transfer molding techniques are used to encapsulatethe entire optical isolator 103 in an opaque package 117 of, e.g., NittoElectric Industrial Co. MP-101S transfer molding compound. Finally, instep 55, all of the leads including leads 95 and 99 are cut from siderails 3, 5 (both shown in FIG. 3) and tie rails 21, 23 (both shown inFIG. 3) and the leads are bent to the desired packaged shape.

FIG. 12 shows an alternate preferred embodiment of the present inventionin which a dielectric sheet 131 is positioned within the isolation gap101 between dice 91, 97. FIG. 13 is a flow chart of the assembly of theoptical isolator 121 shown in FIG. 12 using a number of steps common tothe flow chart of FIG. 4A. In step 151 the sheet 113 shown in FIG. 11 orany other rigid material sheet is attached to the lead 99 using theadhesive layer on the upper surface of the sheet 113. In step 153 asecond dielectric sheet 131 is placed on top of the die 97 touching theend of sheet 113 and adhering to the adhesive layer thereon so that thesheet 131 is angled away from the bond wires thereby avoiding damage tothe bond wires. The tilted angling of sheet 131 creates a wedge shapedcavity between the sheet 131 and the die 97 in which resin 133 iscontained in the desired position for transmitting light to thephotodiode die 97. The sheet 131 is 0.125 inch by 0.125 inch in size andmay be fabricated from a piece of DuPont Company FEP number 200C20material.

In step 155 resin 133, discussed above with reference to FIG. 11, isapplied between the sheet 131 and the die 97 and in step 157 the resinis briefly cured to attain a gel consistency. Performance of thecombination operations of step 47 does not result in damage to the dice91, 97 because the size of the isolation gap 101 is determined by theamount of down set and not by compression of the sheet 131 between thedice 91, 97 as is often done in the prior art. Damage to the bond wiresis avoided since the sheet 131 rests against the sheet 113 and the edgeof die 97 and is thereby tilted at an angle and avoids contact with thebond wires. In step 161, additional resin 133 is applied between thesheet 131 and the die 91. The resin is cured to a gel consistency instep 163 and it is completely cured in step 165 before final steps 53,55 are performed.

The sheet 131 has the effect of increasing the breakdown voltage of theoptical isolator 121 since it has a greater breakdown voltage than doesthe resin 133. It also serves to increase the length of any air gappresent at the interface between the resin 133 and the molding compound117 and minimizes the effect of air bubbles within the resin 133. Theangled presence of the sheet 131 creates a properly positioned and resinfilled wedge shaped cavity which conducts light between the die 91, 97and which also allows a smaller amount of resin 133 to be used so thatthe amount of resin that may wet the leads 95, 99 is reduced. Thisdecrease in lead wetting results in the attainment of superior moisturepenetration resistance properties by the optical isolator 121.

We claim:
 1. An optical isolator which is fabricated upon a lead framehaving two side rails, in accordance with a process having the stepsof:down setting an LED attachment portion of the lead frame and aphotodiode attachment portion of the lead frame to a downset planesubstantially parallel to the plane of the side rails and locatedtherebelow a predetermined downset distance; attaching a photodiode dieto a photodiode lead of the downset photodiode attachment portion;attaching an LED die to an LED lead of the downset LED attachmentportion; rotating one of the attachment portions approximately 180°about an axis in the side rail plane which connects the side rails andthereby positioning the LED die proximate the photodiode die andseparated therefrom by an isolation gap substantially equal to twice thedownset distance minus the sum of the thicknesses of the LED andphotodiode dice; applying a first sheet to a backside of the LED leadopposite the isolation gap; applying a second sheet to a backside of thephotodiode lead opposite the isolation gap; filling a cavity between thefirst and second sheets with an optically transparent material such thatthe isolation gap is filled with the material; creating an encapsulatingbody about the optical isolator; and finishing the lead frame protrudingfrom the body.
 2. An optical isolator fabricated in accordance with theprocess of claim 1, wherein the LED attachment portion is the one of theattachment portions rotated in the rotation step.
 3. An optical isolatorfabricated in accordance with the process of claim 1, wherein thephotodiode attachment portion is the one of the attachment portionsrotated in the rotation step.
 4. An optical isolator fabricated inaccordance with the process of claim 1, wherein the one of theattachment portions is attached to the side rails by hinge pins lying inthe side rail plane and substantially orthogonal to the side rails. 5.An optical isolator fabricated in accordance with the process of claim4, wherein the process further comprises after the rotation step thestep of swaging the hinge pins.
 6. An optical isolator fabricated inaccordance with the process of claim 5, wherein the hinge pins each havea width greater than the thickness of each of the side rails and nogreater than 160% of the side rail thickness.
 7. An optical isolatorfabricated in accordance with the process of claim 6, wherein the firstand second sheets comprise a dielectric material.